The preparation of optically active .alpha.-hydroxycarbonyl compounds from the corresponding .alpha.-ketocarbonyl compounds by means of asymmetric hydrogenation using chiral platinum-metal diphosphine complexes, especially rhodium (I)-1,4-diphosphine complexes, has been extensively investigated. Thus, British Patent 1,592,536 teaches the asymmetric hydrogenation of .alpha.-ketocarbonyl compounds such as e.g. pyruvic acid alkyl ester, phenyl- and alkylglyoxyl acid alkyl ester and ketopantolactone in the presence of rhodium complexes with an optically active 1,4-diphosphine ligand based on a 4-diarylphosphino-2-diarylphosphinomethylpyrrolidine or a 4,5-bis(diarylphosphinomethyl)-1,3-dioxolane which can also be substituted in the 2-position. Other publications such as e.g. published European Patent Applications EP-A 0 158 875, EP-A 0 218 970 and EP-A 0 251 164; Published German Patent Application DE-OS 33 02 697 U.S. Pat. No. 4 343 741; Chemistry Letters (The Chem. Soc. of Japan), (1978) pp. 297-298, (1984) pp. 1603-1606 and (1986) pp. 2061-2064 and Tetrahedron Letters 28 (1987) pp. 3675-3678 concern the further refinement of the above-mentioned method and the production of chiral 1,4-diphosphines as well as of rhodium complexes of the latter. I. OJIMA reports in J. Organomet. Chem. 195 (1985) pp. 239-248 on the mechanism of asymmetric induction by means of cationic Rh-(I) complexes and anionic Rh-(I) complexes formed in situ. In addition to the chiral diphosphine, the complexes contained diolefins and/or the solvent as supporting ligand as well as a coordinating or non-coordinating anion.
The previously known method using rhodium complexes with a chiral 1,4-diphosphane ligand permits the production of .alpha.-hydroxycarbonyl compounds, especially (R)-(-)-pantolactone, in a high yield and in relatively high optical yield; however, the optically active 1,4-diphosphines which have an especially good action and which are based on pyrrolidine can be produced only in multi-stage syntheses--cf. e.g. published European Patent Application EP-A 0 251 164--which makes the method expensive.
Published European Patent Applications EP-A 0 151 282 and EP-A 0 185 882 teach optically active 3,4-bis(diphenylphosphino) pyrrolidines as readily available 1,2-diphosphines for chiral rhodium complexes for asymmetric hydrogenation. These complexes were suggested as catalysts for the asymmetric hydrogenation of .alpha.-acylaminoacrylic acids. However, when using these and other chiral 1,2-diphosphines as ligands in rhodium complexes with diene supporting ligands for the asymmetric hydrogenation of ketopantolactone, it was only possible to obtain (R)-(-)-pantolactone in a low enantiomeric excess; in addition, the conversions were low--cf. reference Example 1. The use of chiral 1,2-diphosphines in complexes containing iridium as central atom instead of rhodium also resulted in the hydrogenation of ketopantolactone in only unsatisfactory conversions and enantiomeric excesses--cf. reference Example 2.
An iridium complex, namely, Ir(DIOP) (COD) Cl (COD=1,5-cyclooctadiene; DIOP=((-)-4,5-bis(diphenylphosphine)-2,2-dimethyl-1, 3-dioxolane) was also included in the investigations of I. OJIMA (cf. supra) on the mechanism of the asymmetric hydrogenation of .alpha.-ketocarbonyl compounds--cf. S. Brunie, J. Mazan, N. Langlois and H. B. Kagan, J. Organomet. Chem. 114, p. 225 (1976). According to the authors it was assumed from the conformation of the Ir complex that it was not likely to be asymmetrically inducing. A moderate optical induction and low conversions were observed by the applicant with the specified complex.